First of all, American Dad isn't a rip-off of Family Guy. The comparison is even more ridiculous than those who claim Family Guy rips off Simpsons. The two families are NOTHING ALIKE. Seth may voice the father figure, but otherwise they have different personalities, as does the rest of the family.
Second, people liking Family Guy can possibly be attributed to its cutaways and general craziness (like Petercopter), which keep the visuals fresh. Comparatively, American Dad is a more straight-forward show, visually, and that may turn some off.
Third, I highly doubt people hate AD because of unlikeable characters, as there have been numerous unlikeable characters in TV history that audiences still clung to. So it must have something to do with people not liking the execution of said AD characters. Personally, I like the characters, but I can see how others would find them mean-spirited.